1. Field of the Invention
The present invention relates to a microwave proximity detector for sensing whether or not an object which causes a loss in a leakage magnetic field from a circuit system having a microwave band oscillator circuit is present in the proximity thereof.
2. Description of the Prior Art
Conventionally, there have been adopted various kinds of proximity detectors or switches, which however are attended with disdavanitges as follows.
In a high frequency proximity switch, a sensing or sense distance in which the switch is effective is small, namely, is at most half the diameter of a sense coil used in the switch (e.g. for a coil having a diameter 12 millimeters, mm, the sense distance is two millimeters). In consequence, when the switch includes a sense coil having a small diameter, there may occur a case where an object to be sensed collides with a sensor head.
In an ultrasonic proximity switch, a reverberation of a mechanical vibration of an ultrasonic vibrator thereof makes it difficult to sense an object existing within a range of at most 5 centimeters, cm (to ten centimeters) from the proximity of the sensor. In production sites such as factories and firms, ultrasonic waves generated, for example, by a collision of work machinery may also cause a wrong operation.
In a photoelectric switch, owing to properties of lights, it is difficult to sense reflection lights from objects such as a transparent object like glass and an object having a matted black surface. Namely, such an object cannot be easily sensed. Furthermore, depending on a degree of dirtiness of a lens of the sensor, dirts or oils thereon, etc., characteristics of the sensor such as a sense distance are disadvantageously varied.
In the microwave proximity sensor of the prior art, since the sense operation is carried out based on a Doppler effect (of a differential output type), there arises a difficulty when a stationary object is to be sensed. Moreover, for sensors of which the intensity of electric field is at least 3.5 f (.mu. V/m), where the frequency is in the unit of gigahertz, GHz and the value is obtained at a distance of three meters from the sensors, an approval is necessary for each sensor in conformity with the Radio Law.